Add releasenote/ to doc8 ignore list
Add releasenote/ to doc8 ignore list

Release note build output was not included in the exclusion list for our
doc8 checks. This causes an issue if someone builds release notes
locally, then runs the pep8 job afterwards.

Also causes an issue for any static rst files included in the source
files.

Change-Id: Ibe0dbbdc8afc6ab3a9a41ab0ca073c567801f2e3
Signed-off-by: Sean McGinnis <961eeb09b6d45516665e15b82e013282c4faf575@gmail.com>

mahak/cinder,openstack/cinder,openstack/cinder,mahak/cinder